I knit a hat and it ended up being too small, so I frogged it and am starting over with more co's. The hat that I frogged used up almost all of 1 skien so I know that this hat will use a little more than 1 skien. My question is this...should I knit until the first skien is gone,even though it will be nearing the completion of my hat, and then start using the second skien or should I knit the hat about half way and then switch to the second skien? Or doesn't it really matter that much?

Doesn't matter. However, if the start of the new skein might fall in an area where it's difficult to join new yarn (if the hat has a lacy area, for instance), I'd start the new one where the join will be the least obvious.